Post-consumer processing of recycled polyethylene terephthalate (PET) to manufacture a variety of useful consumer products such as flower pots and fence posts is well-known. Typically, the recycling process utilizes used PET containers, such as discarded carbonated beverage containers, which are collected, sorted, washed, and separated from contaminants to yield a relatively clean source of RPET. Additionally, the manufacture of imperfect and damaged molded PET products, particularly the blow molded bottles used for containing consumer goods, results in a considerable amount of PET waste which the manufacturers of such products would like to reuse. The RPET produced by conventional recycling processes is generally in ground or flake form, which is thereafter melt processed or further pelletized by-the end user.
RPET is generally subjected to a grinding operation in order to make the material easier to handle and process. Conventional grinding equipment reduces the RPET to about ⅜ inch particles or flakes. The grinding is conducted in a manner to insure that a consistent flake size will be produced, by employing a grate or screen through which the ground material must pass upon exiting the grinder. Although conventional RPET flakes melt processing and pelletizing equipment is designed to handle ⅜ inch flakes, some RPET materials having sizes as large as M inch and as small as ¼ inch are also commercially produced. The bulk density of ⅜ inch flake RPET generally ranges from about 22 to about 35 pounds per cubic foot.
Similarly, RPET and PET pellets are generally formed to a standard, uniform size of about 0.12 inch in diameter. The bulk density of such pellets generally ranges from about 50 to about 58 pounds per cubic foot. Typically, PET and RPET melt processing equipment is designed to accept pellets having the above-mentioned dimensions and physical characteristics.
Conventional methods for comminuting virgin materials to extremely small particle sizes, utilizing ball mills, impact grinding, cryogenic grinding, pulverizing, attrition mills, and the like, are well known. However, none of these methods is effective for reducing RPET derived from used bottles and other containers to extremely small particle sizes. RPET flakes resist communition by conventional methods to particle sizes much smaller than about 500 microns. The polyethylene terephthalate bottles and containers from which RPET flakes are typically derived are manufactured in a manner that imparts great strength and toughness, by incorporating high levels of biaxial orientation into the container sidewalls. This biaxially oriented structure inherently resists fracturing, cracking, splitting, and cutting, which are the conventional processes involved in comminuting operations.
It would be desirable to develop a process for preparing extremely small particles of RPET from standard RPET flakes.